1. Field of the Invention
The present invention relates to heat conductive devices and more particularly to heat pipes wherein a wick is employed to transfer a fluid in an evaporative/condensation cycle through capillary action.
2. Description of the Prior Art
A heat pipe is a closed environment containing a fluid which constantly undergoes an evaporative/condensation cycle. A continuous wick transfers the condensed fluid from the cold portion or condenser to the hot portion or evaporator where the fluid returns to the vapor state. The vapor then moves through the closed environment in that portion not occupied by the wick back to the condenser where it returns to the fluid state. If the heat pipe is to remain operative, the integrity of the cycle must be maintained. Loss of fluid continuity in the wick is the critical item in the cycle. Typically the wick is constructed of a material or by a process which will yield a porous structure comprising a series of intermeshed capillaries. As the fluid in the evaporator enters the gaseous state a high meniscus is formed in these capillaries. The fluid is drawn toward the evaporator by the surface tension of the meniscus. If a dry spot should form across the wick the continuity of this fluid flow may be lost and the cycle broken. Likewise if the "prime" of the wick is lost, the cycle may not begin when heat is applied to the evaporator section of a heat pipe in the static condition.
The foregoing is particularly important when it is desired to incorporate a flexible portion within the heat pipe as may be required in many applications particularly where vibration or body forces i.e. gravity may be a factor. The environmental enclosure of the heat pipe can be made flexible through the use of common materials such as flexible tubing. Providing a flexible wick with adequate performance characteristics which will resist forming discontinuities or changes in performance characteristics is another matter.
The use of helical capillary passages contained within a bellows has been advocated but is limited by surface tension pumping capabilities. Consequently, the total energy that can be dissipated by the heat pipe in a gravity environment is small. Likewise, a wire mesh cut on the bias has been used to bridge the discontinuity in the wick across the flexible portion. In this case the fluid flow capacity of the wire mesh is adequate but the screen wick tends to pull away from the tube wall causing an inefficiency and loss of lifting capacity where the condensor is located above the evaporator.
Another feature which would be desirable in a heat pipe is the ability to provide a simple on/off or "diode" capability. An external artery conducting the working fluid can provide such control. If the capillary is heated, causing the liquid to vaporize within the capillary, the cycle will stop. When the capillary is cooled, vaporization within the artery cannot take place and the cycle will continue.
Therefore, it is an object of the present invention to provide a high performance flexible heat pipe of low flow resistance, high resistivity to loss of prime, and high flow capacity.
It is another object of the present invention to provide a high performance flexible heat pipe which can be constructed of non-special materials.
It is yet another object of the present invention to provide a high performance flexible heat pipe which allows for the cooling or heating of any external arteries contained in the structure.
It is a further object of the present invention to provide a flexible heat pipe that is self priming.
It is a final object of the present invention to provide a flexible heat pipe that eliminates the need for continuous wicks and permits the use of composite wick concepts.
Other objects and advantages of the present invention will become apparent from the figures and specifications which follow.